One essential for the success of a general-purpose language is an accompanying standard library that is rich enough and efficient enough to support the basic, day-to-day tasks common to all programming. Libraries provide the vocabulary with which a language can be used to say something about something. Without a broad common vocabulary, a language community cannot prosper as it might.

This document presents a standard basis library for SML. It is a basis library in the sense that it concerns itself with the fundamentals: primitive types such as integers and floating-point numbers, operations requiring runtime system or compiler support, such as I/O and arrays; and ubiquitous utility types such as booleans and lists. The SML Basis Library purposefully does not cover higher-level types, such as collection types, or application-oriented APIs, such as regular expression matching. The primary reason for limiting the scope in this way is that the design space for these interfaces is large (e.g. choosing between functors and polymorphism as a parameterization mechanism) and, unlike the case with lists and arrays, we do not have many years of common practice to guide the design. It is also the case that the SML Basis Library specification is a substantial document and expanding its scope would make it unwieldy.

The primary purpose of this book is to serve as a reference manual for the Basis Library, describing as clearly and completely as possible the types, values, and modules making up the Library.

Overview

Sockets are an abstraction for interprocess communication (IPC) that were introduced as part of the Berkeley version of Unix in 1982. They have become a de facto standard for network communication and are supported by most major operating systems (including PC systems). The SML Basis Library provides an optional collection of modules for programming with sockets. The interface provided by the Basis follows the C interface for the most part; the major difference is that the SML interface is more strongly typed. In particular, the type system distinguishes between passive and active sockets, between sockets in different domains, and between sockets of different protocols.

The Berkeley Socket API supports two styles of communication: stream sockets provide virtual circuits between pairs of processes, and datagram sockets provide connectionless packet-based communication. In stream-based interactions, the server allocates a master socket that is used to accept connections from clients. The server then listens on the master socket for connection requests from clients; each request is allocated a new socket that the server uses to communicate with that particular client. As the name suggests, stream-based communication is done as a stream of bytes, not as discrete packets. Connectionless communication is more symmetric: messages are sent to a specific port at a specific address. While datagram sockets provide better performance, messages may be lost or received out of order, which requires additional programming by the client. For this reason, stream sockets are more commonly used than datagram sockets.